This article concerns content exclusive to Warcraft RPG and thus unlikely to be canon.
Undercommon is a language known by the half-orc skeletal mage, Chauch-eta. Nothing more is known about this language.
This article or section includes speculation, observations or opinions possibly supported by lore or by Blizzard officials.*
Warcraft Universe Edit
It may be a version of Common or Low Common for the creatures dwelling beneath the earth.
"Undercommon" has an apparent connection to DnD derived generic languages, which apparently can be found in DnD rule books. In other fantasy settings, dark elves speak undercommon.